The present invention relates to an electrophotographic process, and more particularly to a retention type electrophotographic process for forming a plurality of copies of a document from the same and single electrostatic charge image once formed on an electrophotographic photosensitive member.
There have been proposed various processes for forming a plurality of copies of a document. In one known process, a number of copies are duplicated from the same and single electrostatic charge image which has been once formed on a photosensitive drum, by repeatedly effecting development with toner and transfer of a toned image onto successive image receiving papers. Usually such a process is referred to as a retention-type electrophotographic process. In such a process, in order to obtain a number of copies having good image quality, it is necessary to maintain the charge image, once formed on the photosensitive drum, in stable form for a long time period. In known processes, since the latent image is composed of the electrostatic charge applied on an upper surface of the photosensitive member, the latent image might decay or deteriorate due to undesired escape of electrostatic charge through the developing agent and undesired injection of electrostatic charge via the image receiving papers from a biased transfer device. Therefore, the electrostatic charge image could not be retained in stable form on the photosensitive member during the duplication of a plurality of copies, and thus it is difficult to obtain a duplicated image of good quality over a number of copies of the same document.
In order to avoid such a drawback, it is known, from Japanese Patent Application Laid-open Publication No. 72,053/76, to use an electrophotographic photosensitive member 1 comprising an electrically conductive substrate 2, a charge retentive layer 3 made of insulating material and applied on the substrate, and a photoconductive layer 4 applied on the charge retentive layer as illustrated in FIG. 1. At first, a primary electrification of one polarity is effected by means of a corona charger 5 and at the same time the member 1 is irradiated uniformly as shown in FIG. 1A. This irradiation may be effected after the primary electrification. During this step, the charges are trapped across the charge retentive layer 3. Then, as illustrated in FIG. 1B, a secondary electrification of an opposite polarity is effected by means of a corona charger 6, while an image of a document to be duplicated is projected upon the photosensitive member 1. Then in an imagewise bright portion L, the charges are trapped across the charge retentive layer 3, while in an imagewise dark portion D the charges are trapped across the photoconductive layer 4. Next, a uniform exposure is carried out to remove the charges trapped across the photoconductive layer 4 to form an electrostatic charge image as shown in FIG. 1C. This latent image is formed by only the charges trapped across the insulating charge retentive layer 3 and thus is little affected by the development and transfer, so that a number of copies can be formed from the same and single latent image. However, in this known process, there is a drawback in that the resolution of the latent image might be affected by the amount of light used during the uniform exposing step. That is to say, when the light amount is small, the electrostatic contrast becomes smaller although the resolution is increased, whereas when the light amount is large, the resolution becomes smaller although the contrast is improved. Due to the decrease in the resolution, the edge of the image is liable to become obscure. The obscurity at the image edge is assumed to be introduced by the following mechanism. In the secondary electrification together with the imagewise projection shown in FIG. 1B, a positive charge exists on the free surface of the photoconductive layer 4 and a negative charge is trapped in an interface between the photoconductive layer 4 and charge retentive layer 3. In the imagewise bright area L, these positive and negative carriers are cancelled out by means of carrier pairs generated in the photoconductive layer 4 due to the uniform exposure in FIG. 1C. During this process, as shown in FIG. 2, the carrier pairs 7 produced at the image edge are polarized and held along an irregular electric field 8. Therefore, in the bright portion L, the charge trapped in the interface between the charge retentive layer 3 and photoconductive layer 4 is uniformly spread toward the dark portion D. Further, the known photosensitive member 1 has a relatively thick photoconductive layer 4 and thus, the light image of the document is liable to become obscure.
The known electrophotographic process illustrated in FIG. 1 has another drawback in that the contrast of the latent image is greatly influenced by the characteristics of the photoconductive layer 4. That is to say, when it is assumed that the photoconductive layer 4 is of the P-type and the primary electrification is effected in the negative polarity, positive photo-carriers among photo-carrier pairs generated in the photoconductive layer 4 in the uniform exposure step shown in FIG. 1A are neutralized by negative charges on the surface of layer 4, and negative photo-carriers are attracted by the positive carriers which have been induced in the conductive substrate 2 and arrive at the charge retentive layer 3. However, since the photoconductive layer 4 is of the P-type, the mobility of the negative carriers in the photoconductive layer is small, and thus the negative carriers are liable to be trapped in the photoconductive layer 4 to generate a spacial charge. Therefore, the number of charge pairs retained across the charge retentive layer 3 becomes smaller and the contrast of the latent image becomes lower. In order to avoid such a disadvantage, it sould be noted that the resistance of the photoconductive layer 4 can be reduced by effecting the uniform exposure excessively. However, then the concentration of free carriers will be increased, and thus the image quality might be affected during the processes after the simultaneous secondary charging and imagewise exposing step. Further, the lamp for effecting the uniform exposure will consume a greater amount of electric power.
In the case of using a photoconductive layer 4 made of n-type material and effecting primary charge in the negative polarity, during the simultaneously or successively effected primary charging and uniform exposing step, positive photo-carriers among photo-carrier pairs generated in the photoconductive layer 4 are neutralized by negative charges on the surface of photoconductive layer 4, and negative photo-carriers are easily transported in the layer 4 to arrive at the charge retentive layer 3. Therefore, during this process no problems arise. However, in the next step of effecting simultaneously the secondary charging and imagewise exposing shown in FIG. 1B, since positive photo-carriers among photo-carrier pairs generated in the photoconductive layer 4 could not move easily in the n-type photoconductive layer 4, the contrast between the imagewise bright and dark portions might be decreased. This phenomenon will be further enhanced by the fact that during the uniform exposing step illustrated in FIG. 1C, positive photo-carriers among photo-carrier pairs generated in the photoconductive layer 4 in the imagewise dark portion D could not easily move in the photoconductive layer 4.
In the known electrophotographic process shown in FIG. 1, the electrostatic latent image is erased by effecting an A.C. corona charge or making the photoconductive layer 4 into contact with a conductive brush, while the photosensitive member is uniformly irradiated. During this step, if the photoconductive layer 4 has a particular polarity, it is impossible, or at least difficult to erase the latent image completely and residual charges might decrease the resolution of the next electrostatic latent image to be formed.
In order to avoid the above-mentioned various drawbacks, the photoconductive layer 4 may be formed by material having both P and n-type characteristics. However, photoconductive material having both polarity characteristics is not usually found. Moreover, it is quite difficult to treat such photoconductive material so as to control donor and acceptor concentrations to be balanced properly.